This invention relates to a catalyst system for the polycondensation step of polyester production. More particularly, this invention relates to a stable catalyst system for the production of linear polyesters and copolyesters. This invention also relates to a polymerization process employing the catalyst system.
Polymers and copolymers of alkylene terephthalate have found wide spread commercial acceptance. For example, polyesters are used in the manufacture of textile fibers, films, resins, etc.
It is known that polyalkylene terephthalates can be prepared from a suitable alkyl ester of terephthalic acid formed by initially reacting the appropriate alcohol with terephthalic acid. When a methyl ester of terephthalic acid is used as a starting material, it is first reacted with alkylene glycol in the presence of a transesterification catalyst by means of an ester interchange reaction. When terephthalic acid, itself, is used as a starting material, it is subjected to a direct esterification reaction with alkylene glycol in the presence of what is generally called the first stage catalyst additive or ether inhibitor. In either method the resulting reaction product, an ester, is then polycondensed in the presence of a polycondensation catalyst to form polyalkylene terephthalate.
To polymerize a bis(hydroxyalkyl) terephthalate in a reasonable time it is necessary to use a catalyst. Many catalysts have been disclosed for this purpose but it has been found that those giving a rapid production rate also tend to bring about a rapid rate of polymer degradation. Another disadvantage is that many of the known catalysts produce a polymer having a yellowish or gray color. For the manufacture of fibers a color as near white as possible is required and for film making a clear bright polymer is necessary.
It has also long been known in the art that trivalent antimony compounds are excellent polycondensation catalysts. Antimony oxide has been long employed for this use as have various salts and alcohol derivatives such as the alkyl and aryl antimonities, the antimony glycolates, antimony acetates, and antimony oxalate.
However, many of these antimony-containing catalysts produce undesirable side effects, such as a slow rate of reaction, or polymer having a gray color which is undesired in the final terephthalate polymer. It is believed, as discussed in U.S. Pat. No. 3,732,182, that poor rates and gray color formation are caused by a reduction of the antimony catalyst to finely divided metallic antimony. This problem as indicated above has long been known and many attempts have been made to provide a solution.
For example, U.S. Pat. No. 3,484,410 proposes the utilization of trivalent antimony salts of aliphatic hydrocarbon monocarboxylic acids containing at least 12 carbon atoms, while U.S. Pat. No. 3,126,360 discloses the use of various compounds, including benzylic acid and mandelic acid, to be used as a stabilizer or decolorizing agent in the presence of antimony trioxide. U.S. Pat. No. 3,732,182 and U.S. Pat. No. 3,822,239 disclose the utilization of various antimony compounds together with certain alpha-hydroxycarboxylic acids, alpha-beta-dicarboxylic acids, and selected derivatives thereof.
Other antimony-containing compounds have been proposed for uses other than polycondensation catalysts. Thus, U.S. Pat. Nos. 3,407,153; 3,455,995, and 3,471,411 disclose the use of reaction products of alcohols having at least three hydroxyl groups with trivalent antimony compounds as catalysts for the oxyalkylation of polyesters, and as polyurethanes catalysts. Lubricant compositions containing complexes of antimony acid with glycols and polyhydroxybenzenes are taught in U.S. Pat. No. 2,795,551.
Now it has been found in accordance with this invention that selected catalyst systems are efficacious polycondensation catalysts for the preparation of polyalkylene terephthalate.